Dynamics of photoinduced processes in adenine and thymine base pairs

The excited-state dynamics of adenine and thymine dimers and the adenine-thymine base pair were investigated by femtosecond pump-probe ionization spectroscopy with excitation wavelengths of 250-272 nm. The base pairs showed a characteristic ultrafast decay of the initially excited ????* state to an n??* state (lifetime ??????• ≈ 100 fs) followed by a slower decay of the latter with ??n??* ≈ 0.9 ps for (adenine)2, ??n??* = 6-9 ps for (thymine)2, and ??n??* ≈ 2.4 ps for the adenine-thymine base pair. In the adenine dimer, a competing decay of the ????* state via the ????* state greatly suppressed the n??* state signals. Similarities of the excited-state decay parameters in the isolated bases and the base pairs suggest an intramonomer relaxation mechanism in the base pairs.close646